There are many applications wherein light influencing displays such as liquid crystal displays are utilized to advantage. For example, liquid crystal displays find use in digital watches, digital clocks, calculators, pocket-sized television receivers, and various forms of portable games, to name a few.
One common form of display is a twisted nematic liquid crystal display. Displays of this type generally include a pair of facing and spaced apart light transmissive electrode supports or substrates formed of glass, for example. One support carries on its inner or facing surface a first plurality of spaced apart light transmissive pixel electrodes. The other support carries on its inner or facing surface a light transmissive electrode which is common to all of the first plurality of electrodes or a second plurality of light transmissive electrodes with a respective given one facing a corresponding electrode of the first plurality of electrodes. The first plurality of electrodes together with the common electrode or second plurality of electrodes form a corresponding plurality of pixels or picture elements. Twisted nematic liquid crystal material is disposed between the pair of light transmissive electrode supports. The twisted nematic liquid crystal material includes an additive to cause the molecules of the liquid crystal material to have a continuously changing orientation or twist from one electrode to the other. Properly treated alignment layers are formed on the electrodes to promote alignment of the liquid crystal molecules near the electrode surfaces such that the major axes of the molecules are parallel to one another. With twisted nematic liquid crystal material the alignment layers are also treated such that, the axes of the liquid crystal molecules adjacent the first plurality of electrodes are displaced by a quarter turn or 90 degrees relative to the axes of the molecules adjacent the common electrode or second plurality of electrodes. This gives the nematic liquid crystal molecules a continuously changing orientation in the form of helical or twisted displacement of about 90.degree. between the opposing electrodes. The display lastly includes a pair of polarizers on respective opposite sides of the electrode supports.
Depending on the relative alignment of the axes of the polarizers to each other and the display, when the liquid crystal material is in an unenergized state, transmission or absorption of incident light can occur. Upon the application of an electric field across the electrodes, the liquid crystal molecules are rotated into alignment with the field reversing the light transmission state of the display.
The pixels of such displays are generally arranged into M rows and N columns defining a matrix array. Each pixel is addressed using conventional "X-Y" addressing techniques which employ M+N address lines. Thus, each pixel possesses a unique X-Y location in the matrix which may be addressed by applying suitable voltages to a corresponding combination of X and Y addressing lines.
The magnitude of the voltage at which a liquid crystal pixel is switched to a different optical state is generally referred to as the threshold voltage of the liquid crystal material. In the case of large matrix arrays having many pixels, a significant level of electrical cross talk can exist in the addressing circuitry between adjacent pixels. In cases where the voltage threshold is not sufficiently sharp, the cross talk can be sufficient in some cases to energize pixels which are not intended to be addressed. As a result, active matrices have been developed to provide means for isolating each pixel to some degree from circuit cross talk for improving the electrical isolation between adjacent pixels. Such active matrices include nonlinear threshold devices such as diodes or switching devices such as transistors in series with each pixel to enhance the sharpness of the effective threshold of the liquid crystal materials.
While displays of the prior art have found commercial acceptance and applicability, they do suffer from some disadvantages. One example relates to the spacers required between the glass substrates of the displays to accurately control the thickness of the liquid crystal material disposed therebetween. Accurate and uniform control of the thickness of the liquid crystal material is desirable to obtain uniform operating threshold voltages for the pixels. Unfortunately, in the prior art, such spacers are difficult to handle and incorporate into the displays during the manufacture thereof.
In accordance with the prior art, the spacers are incorporated into the display by a technique known as dusting. The spacers are usually in the form of tiny plastic spheres, having a diameter of, for example, 6 microns, or glass cylinders or rods having a diameter of, for example, 6 microns and length of, for example, 10 to 25 microns. The plastic spheres or glass rods are applied to the substrate after the formation of the electrodes and required pixel address lines by a process known as "dusting" which includes subjecting the substrates to a diluted atmosphere of the plastic spheres or glass rods. Static charge then causes the spheres or rods to adhere to the substrate.
As can be appreciated, the above described dusting process results in the sphere or rod spacers to adhere to the substrate in a random pattern. As a result, the spacers can adhere to areas of the substrate which can prove detrimental to desired display operation. For example, the spacers can adhere to the surfaces of the electrodes and thus interfere with the uniform transmission of light through the pixels. The spacers can also adhere to address lines or active matrix devices causing at least one of two known problems. First, if a spacer is adhered to an address line or active device, which are relatively thin in dimension, the spacers can crush the address line or device when the two substrates are brought together and sealed. This can open circuit the address line or destroy the device and render the pixel associated with the crushed address line or device inoperative. Second, even if the address line or device is not crushed, because they have some finite thickness, the thickness of the address line or device will be added to the dimension of the spacers and hence, the substrates will be supported at that point by a distance greater than that desired.
In addition to the foregoing, new and improved liquid crystal materials are being developed for high-speed and low operating voltage displays. Substrate spacings of less than about 3 microns will be required for these new and improved displays. Unfortunately, sphere or rod spacers having such small dimensions are not very uniform in thickness.